How Skywriting Works - putting words in the sky with a smoking plane

There are two ways to do skywriting: The old-fashioned way looks like this and is called "free-style":

This page describes the time it takes for this type of skywriting, along with the visibility:

Each five-letter word takes 10 minutes of carefully choreographed flying that typically includes 20 different precisely timed turns and 15 different bursts of skywriting fluid. It usually requires one-and-a-half hours from take-off to landing...

Each letter is about one mile long and two miles above you, and the average seven-letter message is written across a 10-mile slate of sky...

On a clear day, each letter can be seen for up to 30 miles in any direction from the ground...

And then there is the new "dot matrix printer in the sky" technique using 5 planes flying in formation, which is a lot quicker. A computer turns the smoke on and off in the 5 planes:

The question is, what is the "ink"? What allows a skywriting plane to put white letters in the sky? This is usually smoke created by a smoke system attached to the plane's exhaust pipe, as seen in this test:

You can add a system like this to a radio-controlled plane just as easily:

The following video shows you the equipment involved, which is incredibly simple. All you have to do is inject liquid diesel fuel into the exhaust stream right as it comes out of the engine. You need a tank for the diesel, a way to pump the fuel and a valve that turns the flow on and off: